Post-midnight irregularities in the equatorial regions of topside ionosphere obtained by the CSES satellite

Abstract

Abstract The equatorial post-midnight irregularities, which has been less studied, are investigated using the in situ electron density (Ne) measurements from 2019 to 2021 obtained by the China Seismo-Electromagnetic Satellite (CSES) orbiting in the topside ionosphere. Some results are obtained as following. (1) The equatorial post-midnight irregularities are distributed along the dip equator with a longitude variation pattern of wavenumber 4 and seasonal variation pattern of summer-winter peaks during low solar activity (LSA) period in the topside ionosphere. (2) The longitude variation pattern of post-midnight irregularities is consistent with that of the background Ne with the irregularities concentrated on the peaks of background Ne. (3) The occurrence rate of post-midnight irregularities increases rapidly with increasing solar flux, which can be attributed to the rapid increase in the background Ne during LSA period; moreover, growth rate of occurrence rate is different across different longitudes, with the Pacific and the Atlantic longitudes being the most outstanding regions. (4) The similar characteristics for both post-midnight and post-sunset irregularities suggests they may have similar generation mechanisms, which are supported by the fact that conditions favorable to post-sunset irregularity generation can appear at midnight hours during LSA period. Since there have been far fewer post-midnight irregularity studies compared to post-sunset irregularity studies, much more work is still required to understand its generation mechanisms. The large number of post-midnight Ne measurements from the CSES satellite provides a good opportunity for this study, which can help to understand the post-midnight ionospheric dynamical processes and their variations with solar activity, as well as to develop a more comprehensive irregularity forecast model.